qemu-storage-daemon: add --pidfile option

Daemons often have a --pidfile option where the pid is written to a file
so that scripts can stop the daemon by sending a signal.

The pid file also acts as a lock to prevent multiple instances of the
daemon from launching for a given pid file.

QEMU, qemu-nbd, qemu-ga, virtiofsd, and qemu-pr-helper all support the
--pidfile option. Add it to qemu-storage-daemon too.

Reported-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20210302142746.170535-1-stefanha@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

docs/tools/qemu-storage-daemon.rst

@@ -118,6 +118,20 @@ Standard options:
   List object properties with ``<type>,help``. See the :manpage:`qemu(1)`
   manual page for a description of the object properties.
 
+.. option:: --pidfile PATH
+
+  is the path to a file where the daemon writes its pid. This allows scripts to
+  stop the daemon by sending a signal::
+
+    $ kill -SIGTERM $(<path/to/qsd.pid)
+
+  A file lock is applied to the file so only one instance of the daemon can run
+  with a given pid file path. The daemon unlinks its pid file when terminating.
+
+  The pid file is written after chardevs, exports, and NBD servers have been
+  created but before accepting connections. The daemon has started successfully
+  when the pid file is written and clients may begin connecting.
+
 Examples
 --------
 Launch the daemon with QMP monitor socket ``qmp.sock`` so clients can execute

storage-daemon/qemu-storage-daemon.c

@@ -59,6 +59,7 @@
 #include "sysemu/runstate.h"
 #include "trace/control.h"
 
+static const char *pid_file;
 static volatile bool exit_requested = false;
 
 void qemu_system_killed(int signal, pid_t pid)
@@ -115,6 +116,8 @@ static void help(void)
 "                         See the qemu(1) man page for documentation of the\n"
 "                         objects that can be added.\n"
 "\n"
+"  --pidfile <path>       write process ID to a file after startup\n"
+"\n"
 QEMU_HELP_BOTTOM "\n",
     error_get_progname());
 }
@@ -126,6 +129,7 @@ enum {
     OPTION_MONITOR,
     OPTION_NBD_SERVER,
     OPTION_OBJECT,
+    OPTION_PIDFILE,
 };
 
 extern QemuOptsList qemu_chardev_opts;
@@ -178,6 +182,7 @@ static void process_options(int argc, char *argv[])
         {"monitor", required_argument, NULL, OPTION_MONITOR},
         {"nbd-server", required_argument, NULL, OPTION_NBD_SERVER},
         {"object", required_argument, NULL, OPTION_OBJECT},
+        {"pidfile", required_argument, NULL, OPTION_PIDFILE},
         {"trace", required_argument, NULL, 'T'},
         {"version", no_argument, NULL, 'V'},
         {0, 0, 0, 0}
@@ -289,6 +294,9 @@ static void process_options(int argc, char *argv[])
                 qobject_unref(args);
                 break;
             }
+        case OPTION_PIDFILE:
+            pid_file = optarg;
+            break;
         case 1:
             error_report("Unexpected argument");
             exit(EXIT_FAILURE);
@@ -299,6 +307,27 @@ static void process_options(int argc, char *argv[])
     loc_set_none();
 }
 
+static void pid_file_cleanup(void)
+{
+    unlink(pid_file);
+}
+
+static void pid_file_init(void)
+{
+    Error *err = NULL;
+
+    if (!pid_file) {
+        return;
+    }
+
+    if (!qemu_write_pidfile(pid_file, &err)) {
+        error_reportf_err(err, "cannot create PID file: ");
+        exit(EXIT_FAILURE);
+    }
+
+    atexit(pid_file_cleanup);
+}
+
 int main(int argc, char *argv[])
 {
 #ifdef CONFIG_POSIX
@@ -326,6 +355,13 @@ int main(int argc, char *argv[])
     qemu_init_main_loop(&error_fatal);
     process_options(argc, argv);
 
+    /*
+     * Write the pid file after creating chardevs, exports, and NBD servers but
+     * before accepting connections. This ordering is documented. Do not change
+     * it.
+     */
+    pid_file_init();
+
     while (!exit_requested) {
         main_loop_wait(false);
     }